Foundations and Applications

Joseph John Thomson

ENGLISH PHYSICIST 1856–1940

Joseph John Thomson, always known as "J. J.," was born in
Manchester, England, on December 18, 1856. His fame derives primarily from
his discovery of the electron in 1897. He studied physics and mathematics,
first in Manchester, and in 1876 went to Trinity College, Cambridge
University, and never left. He graduated in 1880 and in 1884 succeeded
Lord Rayleigh as professor of physics and director of the Cavendish
Laboratory. (When he retired from Cavendish in 1919 he passed the baton to
Ernest Rutherford). Thomson made Cambridge a world center for atomic
physics. He won the Nobel Prize for physics in 1906 for his work on the
electron, and seven of his research associates went on to win Nobel
Prizes. The electron could almost be said to have been a family heirloom,
as his son, George Paget Thomson, won the Nobel Prize for physics (in
1937) for showing the wave nature of the electron.

His early work in electromagnetism led him to say, in 1893: "There
is no other branch of physics which affords us so promising an opportunity
of penetrating the secret of electricity." He turned his attention
to cathode rays, and his subsequent investigations of these rays led him
to the idea that they consisted of bodies smaller than atoms.
Thomson's main contribution to science was the clear identification
of the electron and its characterization as an elementary, subatomic
particle in 1897. He showed that cathode rays were deflected by both
magnetic and electric fields, and he was able to measure a cathode
ray's charge/mass (
e/m
) ratio. Figure 1 is a schematic of diagram of his apparatus, showing how
a beam of electrons can be subjected to opposing electric and magnetic
fields, which can be adjusted until their effects balance. This enabled
him to estimate the mass of the electron as 1/1,837 of a hydrogen atom.
The electron was the first subatomic particle to be discovered, and he
made the inspired guess that it was a universal constituent of matter. He
said: "… [W]e have in the cathode rays matter in a new
state, a state in which the subdivision of matter is carried very much

Figure 1. Basic features of Thomson's apparatus.

further than in the ordinary gaseous state: a state in which all matter
… is of one and the same kind; this matter being the substance from
which all the chemical elements are built up." He announced his
discovery in the course of a public lecture at the Royal Institution in
London, on April 30, 1897, in which he said: "Could anything at
first sight seem more impractical than a body which is so small that its
mass is an insignificant fraction of the mass of an atom of
hydrogen?" Thomson referred to electrons as
"corpuscles" (even in his 1906 Nobel lecture).

English physicist Sir Joseph John Thomson, recipient of the 1906
Nobel Prize in physics, "in recognition of the great merits of
his theoretical and experimental investigations on the conduction of
electricity by gases."

Thomson devised the famous plum pudding model of the atom, in which
electrons were compared to negative plums embedded in a positively charged
pudding. The idea was wrong, and his successor at Cambridge, Ernest
Rutherford, was soon to develop the
nuclear
model of the atom.

Thomson investigated positive rays, which consist of ionized atoms,
beginning in 1906. He was able to use a combination of electric and
magnetic fields to separate different charged atoms of elements on the
basis of their charge/mass ratios. He was the first to show that neon
contained two atoms of slightly different masses, in a paper published in
1913. As part of the conclusion of the paper he wrote: "There can,
therefore, I think, be little doubt that what has been called neon is not
a simple gas but a mixture of two gases, one of which has an atomic weight
about 20 and the other about 22. The parabola due to the heavier gas is
always much fainter than that due to the lighter, so that probably the
heavier gas forms only a small percentage of the mixture." The two
forms of neon were called
isotopes
by Frederick Soddy. One of Thomson's students, Frederick Aston,
developed Thomson's idea of multiple species of an element, and in
1919 Aston produced the first mass spectrograph (an instrument that
determined isotopic ratios), ancestor of today's mass spectrometer.

Thomson was a great advocate of pure research, in contrast to applied
research, declaring: "[R]esearch in applied science leads to
reforms, research in pure science leads to revolutions, and revolutions,
whether political or industrial, are exceedingly profitable things if you
are on the winning side." Thomson was knighted in 1908 and received
many awards and honors. He died during the early part of World War II, on
August 30, 1940, and is buried in Westminster Abbey near Sir Isaac Newton,
in recognition of his great contributions to science.